FIELD OF THE INVENTION
The invention relates to a device and a method for collecting and cooling a reactor melt or reactor-meltdown products from a reactor pressure vessel. The device and the method are usable particularly in EPR pressurized water reactors.
In nuclear reactors, safety considerations so far have assumed that failure of the reactor pressure vessel need not be feared, because of the choice of material and the dimensions. Recently, however, in the course of more-intensive safety investigations of nuclear energy utilization, the situation in which a reactor pressure vessel "fails", however unlikely that may be, has also been studied. In particular a new reactor type, the European pressurized water reactor or EPR has been based on such considerations. In contrast to the safety philosophy used heretofore, in that reactor type a nuclear meltdown accident, a so-called MCA (Maximum Credible Accident), is not flatly excluded as a possibility.
Investigations have also been made as to whether or not steam explosions might occur during a core meltdown, and whether or not the water vapor suddenly produced in such a critical phase might not cause the pressure vessel to collapse. There is no question that such accidents, however theoretical they might be, must be made controllable.
In a hypothetical serious accident in a nuclear power plant with a water-cooled reactor, it is accordingly assumed that the reactor core will melt. Subsequently, reactor-meltdown products escape from the bottom of the reactor pressure vessel into the shielding pit of the reactor containment. In order to enable such an accident to be controlled, suitable structural provisions must be made to prevent the meltdown products, possibly escaping at excess pressure from the reactor pressure vessel and collecting in the region of the bottom of the nuclear reactor plant, from causing failure of the containment.
A nuclear reactor plant that is known from German Patent 28 40 086, corresponding to UK Patent Application GB 2 030 347 A, for instance, has a collecting device for a reactor core which is melting down. A vertical drain channel is provided below the shielding pit surrounding the reactor pressure vessel. That drain channel passes through the containment and leads into a meltdown pit disposed below the containment.
There the reactor-meltdown products emerging from the reactor pressure vessel are distributed over an absorber bed formed by a steel tub filled with water-free materials. After the steel tub melts, the meltdown products reach the bottom of the meltdown pit. The bottom and the side walls of the meltdown pit are water-cooled, and thus the meltdown products gradually solidify.
In the nuclear reactor plant known from German Published, Non-Prosecuted Patent Application 29 25 680, corresponding to UK Patent Application GB 2 052 133 A, a collecting tub for receiving the meltdown products is also provided and is located below the level of the reactor foundation. The collecting tub is not located directly below the reactor core but rather is located next to it and is connected to the bottom of the reactor building through a chute that ends horizontally above the collecting tub.
In those known structures for receiving the reactor-meltdown products, under unfavorable conditions it may not be possible to prevent a relatively large quantity of water from invading the collecting chamber before or during the outflow of the meltdown products. The result thereof can be that the meltdown products strike the water at high speed and are fragmented there, causing a very forceful, sudden development of steam. The invention is based on the recognition that the quantity of water struck by the reactor-meltdown products should be as low as possible.
A collecting device for reactor-meltdown products is known from FIG. 1 of the publication entitled: Emerging Nuclear Energy Systems 1989, Icenes 89, Karlsruhe, July 3-6, Proceedings of the Fifth International Conference on Emerging Nuclear Systems, pp. 19-24. In that device, a cooled collecting tub is disposed inside the containment directly beneath the reactor pressure vessel, and in that tub the meltdown products spread out over a large area and can cool down in direct contact with water. The steam produced by the heat of decay of the meltdown products condenses in the upper part of the steel hull of a containment, and from there flows back to the collecting device for reactor-meltdown products. In that known nuclear reactor plant as well, it cannot be precluded that even before the failure of the reactor pressure vessel, sump water may collect in the collecting tub and in the shielding pit, so that at the moment of reactor pressure vessel failure there is the risk that the escaping meltdown products will strike the water surface directly and at high speed. Once again, that can cause severe fragmentation of the meltdown products and very forceful steam production, which threatens the containment.
European Published Patent Application 0 392 604 A1, corresponding to U.S. Pat. No. 5,057,271, discloses a collecting and cooling device in which the water inlet is located beneath the reactor pressure vessel. In the case of a meltdown, the reactor-meltdown products and portions of the reactor pressure vessel and its built-in fixtures would drop directly into the water bath. From the standpoint of effective cooling and the aversion of steam explosions, that is not expedient. What is sought is instead a cooling process in which the outflowing reactor-meltdown products will not meet a relatively large quantity of water.